1. Field of the Invention
The present invention relates to display devices such as an organic EL (Electro Luminescence) image display device and a liquid crystal display device that are provided with a pixel matrix in which a plurality of pixels are arranged in matrix form, and more particularly to an organic EL image display device and liquid crystal display device for receiving digital image signals and driving each pixel of the pixel matrix, as well as to a method of driving such devices.
2. Description of the Related Art
Owing to their superior display characteristics, active matrix-type liquid crystal display devices in which TFT (Thin-Film Transistors), which are active elements, are provided in each pixel have become the mainstream in current liquid crystal display devices. Among these active matrix liquid crystal display devices, the current trend is towards devices that employ polysilicon TFTs as the active elements. This is because, when polysilicon TFTs are used for each pixel, in addition to the pixel TFTs, the gate drivers for driving the scanning lines that are connected to the gates of the pixel TFTs as well as the data driver for driving the signal lines that are connected to the source terminals of the pixel TFTs can be produced at the same time on the glass substrate on which the pixels are fabricated. This enables a dramatic reduction of the number of connection terminals between the liquid crystal display device and outside circuits, a miniaturization of the liquid crystal display device module, a simplification of external circuits, and further, a decrease in cost. However, polysilicon TFT exhibit a high degree of variation of characteristics in comparison with single-crystal silicon transistors, and a highly accurate analog circuit is therefore difficult to realize. As a result, a data driver that handles image signals, which are analog signals, frequently consists of simple switches for sampling signals that are supplied from outside circuits and scanning circuits for controlling these switches. Voltage that is applied to the liquid crystal element must be ±5 V with respect to the opposite electrode, and the analog image signal that is supplied to the liquid crystal display device therefore has voltage amplitude of about 10 V. In addition, the frequency of the analog image signal is relatively high from several MHz to several tens of MHz and therefore is a large burden on the outside circuits that supply the image signal to the liquid crystal display device.
For these reasons, many attempts have been made to achieve a simplification of the outside circuits and a lower cost by supplying image signals to the liquid crystal display device in the form of digital data and then converting to analog signals in the liquid crystal display device. More specifically, a DAC (Digital-Analog Converter) is provided in the data driver to enable the liquid crystal display device to handle digital image signals. FIG. 1 shows a representative example of a DAC that is used in a data driver of such a liquid crystal display device. DAC 50 shown in FIG. 1 is an equivalent representation of the DAC (Digital-Analog Converter) for a polysilicon TFT data driver that was reported in “Y. Matsueda (SID (Society for Information Display), 1996 Digest, pp. 21-24.)”. This DAC 50 is a variation of a device generally referred to as a capacitor-array DAC and realizes digital/analog conversion by means of charge redistribution between a binary-weighted capacitor array C1-Cn, auxiliary capacitor C0, and load capacitance (parasitic capacitance) Cd of the signal line that is the load of DAC 50. In this construction, DAC 50 can consist of capacitances C1-Cn and switches, and has the merit of enabling DAC at relatively high accuracy despite the use of polysilicon TFTs, which exhibit a relatively high degree of variation in characteristics between elements.
Nevertheless, this method has two problems. First, the output of DAC 50 that is described here, in contrast with a typical capacitor-array DAC, is supplied directly to signal lines, which is the load, without passing through an analog amplifier, and the output voltage is therefore lower than the voltage that is applied to capacitor-array C1-Cn. Solving this problem necessitates the incorporation of a capacitor array having a capacitance of the same or a higher level of capacitance than the load capacitance Cd of the signal line, which is the load. This case creates a new problem in that the circuit area of DAC 50 increases.
The second problem is that raising the resolution of DAC 50 results in a simultaneous increase in the circuit area. This problem occurs because the resolution (number of digital data bits) is equal to the number of capacitor arrays.
The above-described problems occur not only in a liquid crystal display device but also similarly occur in an active matrix EL (Electro Luminescence) image display device.